A unique fiberoptic pH probe was designed by the Biomedical Engineering and Instrumentation Branch which permits monitoring of intramyocardial pH. Studies are in progress to demonstrate that this probe reflects true pH values as compared to accepted techniques. The probe has been shown to read known buffers and arterial blood gas samples as confirmed by an IL, Instrumentation Laboratory pH/Blood Gas Analyzer. The stability of the pH readings over hours has been demonstrated. The implanted probe records a normal pH of 7.4 prior to partial coronary artery occlusion in open check anesthetized dogs. Intramyocardial pH falls with ischemia and recovers quickly with reperfusion. Metabolic and respiratory and base changes are effected and reflected by the intramyocardial pH probe;. A pH gradient across the heart muscle wall has also been demonstrated.